The heart of an active electronic device is the transistor. Real progress in electronics has been paced by improvements in this basic device and the ability to pack more and more of them onto a single silicon or gallium arsenide wafer. In order for those active devices (or integrated circuits) to perform, they must be electrically connected to other devices. The devices must also be protected from the surrounding environment, from physical abuse, and from overheating. These necessary connections and protections are best provided by encasing the active devices within a hermetic package.
The rapid advance of integrated circuit technology in the past decade has not been matched by similar improvements in needed packaging capability. Thus, the lag in technology required to properly package the active device has actually limited the use of the device.
The largest volume of today's integrated circuits is contained in plastic packaging. In those applications where high reliability and long life have been demanded, packaging systems consisting predominantly of Al.sub.2 O.sub.3 and Al.sub.2 O.sub.3 /glass mixtures have customarily been used. Several drawbacks have been experienced in the use of packages containing Al.sub.2 O.sub.3, however, as will be explained below.
As the integrated circuit device becomes more complex, i.e., comprising more active elements, a higher proportion of the system signal response time is required to transmit signals between chips, to other integrated circuit devices, etc. Packaging high speed devices demands close control of noise, impedance, resistivity, and temperature rise.
Improved interconnect signal speed and integrity can be achieved by shortening the signal path between chips, by improved ceramic electrical properties, viz., lower dielectric constant and lower dissipation factor, by reducing the resistance of the signal conductor, and by the reduction of noise. The closer spacing of chips requires very fine and closely spaced signal lines and multi-layer packages with fine internal connections (vias). Surface smoothness and flatness, together with dimensional control, become critical for fine line metallization and via registration.
Whereas Al.sub.2 O.sub.3 can be ground to a smooth finish, the high shrinkage thereof (.apprxeq.18%) and difficulty in machining present problems for high density via registration. The high dielectric constant of Al.sub.2 O.sub.3 ( 10) restricts line spacing (development of cross talk and noise) and also slows the signal itself. A major drawback witnessed in the use of Al.sub.2 O.sub.3 packaging resides in the need for utilizing high temperature metals such as molybdenum or tungsten because of the very high co-firing temperature required for sintering (.gtoreq.1500.degree. C. While the resistivities of those metals are relatively low, they are significantly higher than those of silver, copper, and gold and, of course, necessitate gold plating prior to soldering.
Therefore, the objective of the present invention was to develop a dielectric material for use in packaging active electronic devices exhibiting properties constituting a significant improvement over Al.sub.2 O.sub.3. The goal of the research was, therefore, to develop a material demonstrating the following characteristics: (1) compatibility with copper, silver, and gold metallization; (2) the capability of being co-fired at temperatures between about 850.degree.-1000.degree. C.; (3) a dielectric constant less than about 6; (4) a smooth, flat surface without additional processing; and (5) preferably a coefficient of thermal expansion compatible with silicon (.apprxeq.35.times.10.sup.-7 /.degree.C.) or gallium arsenide (.apprxeq.60.times.10.sup.-7 /.degree.C.).
The present invention resulted from research directed to developing surface-nucleated glass-ceramic frits as replacements for Al.sub.2 O.sub.3. That research focussed primarily in the fabrication of multi-layer packages by processing glass-ceramic frits into thin dielectric sheets through tape casting.
Glass-ceramics had their genesis in U.S. Pat. No. 2,920,971. As explained there, glass-ceramics are conventionally prepared through the controlled crystallization of precursor glass bodies through three general steps: (1) a glass forming batch, commonly containing a nucleating agent, is melted; (2) that melt is cooled below the transformation range thereof and simultaneously shaped into a glass body of a desired geometry; and (3) that glass body is subjected to a predetermined heat treatment to cause the generation of crystals in situ. Frequently, the heat treatment of the glass body is applied in two stages; viz., the body is initially heated to a temperature within or slightly above the transformation range to cause the development of nuclei therein, and, subsequently, the nucleated body is heated to a temperature approaching or even exceeding the softening point of the glass to grow crystals on the nuclei. (The transformation range has been defined as the temperature at which a molten mass is deemed to have become an amorphous solid; that temperature being generally considered as residing in the vicinity of the glass annealing point.)
The rate of crystal growth is a function of temperature; i.e., crystal development becomes more rapid as the temperature is raised. This circumstance permits a degree of control over the concentration of crystallization in the final product. Customarily, the precursor glass body is heat treated sufficiently to yield highly crystalline articles, i.e., greater than 50% by volume crystalline and, frequently, closely approaching 100%. The normally high crystallinity of glass-ceramics, i.e., greater than 50% by volume crystalline and, frequently, closely approaching 100%, leads to two results: (1) the physical properties displayed by glass-ceramics more nearly resemble those of the crystal phase than those of the residual glass; and (2) the composition of the residual glass is quite unlike that of the parent glass, since the components of the crystal phase will have been removed therefrom; so, consequently, the physical properties demonstrated by the residual glass will be different from those of the precursor glass. Finally, because the crystals of a glass-ceramic are encompassed within a residual glassy matrix, the surface of a glass-ceramic body is smooth and the interior portion thereof non-porous and free of voids.
Whereas most glass-ceramic articles have been prepared from precursor glass compositions containing a component specifically designed to perform as a nucleating agent, glass-ceramic bodies have been successfully produced by utilizing surface nucleation in the sintering together and consolidating of very fine glass powders. Such bodies can exhibit the contours and physical properties demonstrated by glass-ceramics prepared in the more conventional process.